1MQ_RECEIVE(P) POSIX Programmer's Manual MQ_RECEIVE(P)
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6 mq_receive, mq_timedreceive - receive a message from a message queue
7 (REALTIME)
8
10 #include <mqueue.h>
11 ssize_t mq_receive(mqd_t mqdes, char *msg_ptr, size_t msg_len,
13 unsigned *msg_prio);
14 #include <mqueue.h>
19 #include <time.h>
20 ssize_t mq_timedreceive(mqd_t mqdes, char *restrict msg_ptr,
21 size_t msg_len, unsigned *restrict msg_prio,
22 const struct timespec *restrict abs_timeout);
23
26 The mq_receive() function shall receive the oldest of the highest pri‐
27 ority message(s) from the message queue specified by mqdes. If the size
28 of the buffer in bytes, specified by the msg_len argument, is less than
29 the mq_msgsize attribute of the message queue, the function shall fail
30 and return an error. Otherwise, the selected message shall be removed
31 from the queue and copied to the buffer pointed to by the msg_ptr argu‐
32 ment.
33 If the value of msg_len is greater than {SSIZE_MAX}, the result is
35 implementation-defined.
36 If the argument msg_prio is not NULL, the priority of the selected mes‐
38 sage shall be stored in the location referenced by msg_prio.
39 If the specified message queue is empty and O_NONBLOCK is not set in
41 the message queue description associated with mqdes, mq_receive() shall
42 block until a message is enqueued on the message queue or until
43 mq_receive() is interrupted by a signal. If more than one thread is
44 waiting to receive a message when a message arrives at an empty queue
45 and the Priority Scheduling option is supported, then the thread of
46 highest priority that has been waiting the longest shall be selected to
47 receive the message. Otherwise, it is unspecified which waiting thread
48 receives the message. If the specified message queue is empty and
49 O_NONBLOCK is set in the message queue description associated with
50 mqdes, no message shall be removed from the queue, and mq_receive()
51 shall return an error.
52 The mq_timedreceive() function shall receive the oldest of the highest
54 priority messages from the message queue specified by mqdes as
55 described for the mq_receive() function. However, if O_NONBLOCK was not
56 specified when the message queue was opened via the mq_open() function,
57 and no message exists on the queue to satisfy the receive, the wait for
58 such a message shall be terminated when the specified timeout expires.
59 If O_NONBLOCK is set, this function is equivalent to mq_receive().
60 The timeout expires when the absolute time specified by abs_timeout
62 passes, as measured by the clock on which timeouts are based (that is,
63 when the value of that clock equals or exceeds abs_timeout), or if the
64 absolute time specified by abs_timeout has already been passed at the
65 time of the call.
66 If the Timers option is supported, the timeout shall be based on the
68 CLOCK_REALTIME clock; if the Timers option is not supported, the time‐
69 out shall be based on the system clock as returned by the time() func‐
70 tion.
71 The resolution of the timeout shall be the resolution of the clock on
73 which it is based. The timespec argument is defined in the <time.h>
74 header.
75 Under no circumstance shall the operation fail with a timeout if a mes‐
77 sage can be removed from the message queue immediately. The validity
78 of the abs_timeout parameter need not be checked if a message can be
79 removed from the message queue immediately.
80
82 Upon successful completion, the mq_receive() and mq_timedreceive()
83 functions shall return the length of the selected message in bytes and
84 the message shall be removed from the queue. Otherwise, no message
85 shall be removed from the queue, the functions shall return a value of
86 -1, and set errno to indicate the error.
87
89 The mq_receive() and mq_timedreceive() functions shall fail if:
90 EAGAIN O_NONBLOCK was set in the message description associated with
92 mqdes, and the specified message queue is empty.
93 EBADF The mqdes argument is not a valid message queue descriptor open
95 for reading.
96 EMSGSIZE
98 The specified message buffer size, msg_len, is less than the
99 message size attribute of the message queue.
100 EINTR The mq_receive() or mq_timedreceive() operation was inter‐
102 rupted by a signal.
103 EINVAL The process or thread would have blocked, and the abs_timeout
105 parameter specified a nanoseconds field value less than zero or
106 greater than or equal to 1000 million.
107 ETIMEDOUT
109 The O_NONBLOCK flag was not set when the message queue was
110 opened, but no message arrived on the queue before the specified
111 timeout expired.
112 The mq_receive() and mq_timedreceive() functions may fail if:
115 EBADMSG
117 The implementation has detected a data corruption problem with
118 the message.
119 The following sections are informative.
122
124 None.
125
127 None.
128
130 None.
131
133 None.
134
136 mq_open() , mq_send() , mq_timedsend() , msgctl() , msgget() , msgrcv()
137 , msgsnd() , time() , the Base Definitions volume of
138 IEEE Std 1003.1-2001, <mqueue.h>, <time.h>
139
141 Portions of this text are reprinted and reproduced in electronic form
142 from IEEE Std 1003.1, 2003 Edition, Standard for Information Technology
143 -- Portable Operating System Interface (POSIX), The Open Group Base
144 Specifications Issue 6, Copyright (C) 2001-2003 by the Institute of
145 Electrical and Electronics Engineers, Inc and The Open Group. In the
146 event of any discrepancy between this version and the original IEEE and
147 The Open Group Standard, the original IEEE and The Open Group Standard
148 is the referee document. The original Standard can be obtained online
149 at http://www.opengroup.org/unix/online.html .
150IEEE/The Open Group 2003 MQ_RECEIVE(P)